The present invention pertains to an apparatus and method providing substantially two-dimensionally uniform irradiation of large areas with a high level of irradiation. More particularly, the present invention pertains to an apparatus for and a method of uniformly projecting a high level of irradiation onto a large planar target surface so as to uniformly treat the surface.
Various manufacturing processes include treating a planar surface by irradiating the surface with, for example, ultraviolet light or other irradiation. The irradiation treatment may be related to curing, purification, disinfection, advanced oxidation or some other procedure. By way of example, manufacturing of printed circuit boards frequently involves forming conductive paths by a photoresist process in which a board treated with a photoresist in a desired pattern is irradiated as a part of a process to remove material from specified areas on the board. Similarly, in some printing processes a printed pattern is cured by irradiating the pattern. Obtaining a high quality, uniform product requires irradiating a two-dimensionally uniform high level of irradiation over the entire target area. Otherwise irregularities in the finished product may result.
Existing devices often expose the central area of the irradiated surface to more irradiation than the edge areas of the surface. The areas of high irradiation may receive more than the desired level, possibly causing damage, while the areas of low irradiation may be undertreated. This problem is even greater in the treating of relatively large planar surfaces when a relatively high level of irradiance is needed.
Various techniques have been used in the past to control the uniformity of irradiation of planar target surfaces. By way of example, U.S. Pat. No. 4,010,374 discloses an ultraviolet light processor including a primary light source which exposes a target surface on a work piece to ultraviolet light with the ultraviolet flux incident per unit area of the target surface greater at the central region of the surface than at edges of the surface, and a secondary light source which is positioned in a different plane than the primary light source and which exposes the target surface to ultraviolet light with the ultraviolet flux incident per unit area of the surface greater at the edge areas of the target surface than at the central region. Not only is such an ultraviolet light processor complex and expensive to manufacture and to operate, but also it is difficult to control in a manner that maintains the ultraviolet irradiation received at the edge areas of the target surface from the secondary source at substantially the same level as the ultraviolet irradiation received at the central area of the target surface from the primary source. In addition, the multiple irradiation sources often result in excessive heat which must be removed to avoid deformation, color change, or other damage to the irradiated surface.
U.S. Pat. No. 4,276,479 discloses a tunnel type irradiation chamber with a plurality of cylindrical ultraviolet lenses through which an object to be treated is conveyed. Two sets of irradiation sources, providing light of two different wavelengths, are within the chamber, providing light in two stages. Not only is this apparatus complex to control, but also it frequently does not provide uniform irradiation distribution on the object surface.
U.S. Pat. No. 4,348,015 shows an irradiation projection system including complex lenses in order to provide uniform irradiance. Numerous other systems have been attempted. These generally are complex and expensive, both to construct and to operate. Even so, they generally have difficulty in achieving uniform irradiance, particularly two-dimensionally uniform irradiance.
The present invention is an apparatus for and a method of providing substantially two-dimensionally uniform irradiation of planar areas with a high level of irradiation. In accordance with the present invention, an elongated source of irradiation, such as an elongated discharge tube, is arranged within an elongated elliptical reflecting trough, with the irradiation source spaced from the focal axis within the trough. The trough longitudinal edges define a first plane substantially perpendicular to the trough major axis. First and second reflectors extend from the trough longitudinal edges. Third and fourth reflectors extend from the ends of the trough. The first, second, third, and fourth reflectors extend to reflector outer edges which define the plane of the target surface. That target surface plane is substantially parallel with the first plane. Preferably, the first reflector includes a first portion extending from one longitudinal edge of the trough and angled toward the target surface at an angle greater than 0xc2x0 with respect to the first plane, and a second portion extending from the first portion and angled toward the target surface at an angle greater than 90xc2x0 with respect to the first plane. Similarly, the second reflector preferable includes a first portion extending from the other trough longitudinal edge and angled toward the target surface at an angle greater than 0xc2x0 with respect to the first plane, and a second portion extending from the first portion and angled toward the target surface at an angle greater than 90xc2x0 with respect to the first plane. Preferably, also, the first and second reflectors are cold reflectors.
An elongated central reflector is positioned on the trough major axis, outside the trough and has a first planar portion lying in a plane substantially parallel with the target surface plane. The first planar portion longitudinal axis extends substantially parallel with the longitudinal axis of the irradiation source. Preferably, the central reflector includes second and third planar portions extending from the longitudinal edges of the first planar portion. Each of the second and third planar portions is angled toward the target surface at an angle greater than 0xc2x0 with respect to the plane of the first planar portion. Preferably, also, the central reflector is a translucent mirror.
In a preferred embodiment, the second portion of the first reflector has an ultraviolet light reflectance xcfx81uv1, the second portion of the second reflector has an ultraviolet light reflectance xcfx81uv2, the third reflector has an ultraviolet light reflectance xcfx81uv3, the fourth reflector has an ultraviolet light reflectance xcfx81uv4, the first portion of the first reflector has an ultraviolet light reflectance xcfx81uv5, the first portion of the second reflector has an ultraviolet light reflectance xcfx81uv6, the translucent reflector first planar portion has an ultraviolet light reflectance xcfx81uv7, the translucent reflector second planar portion has an ultraviolet light reflectance xcfx81uv8, and the translucent reflector third planar portion has an ultraviolet light reflectance xcfx81uv9, and xcfx81uv1=xcfx81uv2=xcfx81uv3=xcfx81uv4 greater than xcfx81uv5=xcfx81uv6 greater than xcfx81uv7 greater than xcfx81uv8=xcfx81uv9. In a particularly preferred embodiment, xcfx81uv1=xcfx81uv2=xcfx81uv3=xcfx81uv4=100%; xcfx81uv5=xcfx81uv6=90%, xcfx81uv7=65%, and xcfx81uv8=xcfx81uv9=60%, surface reflection losses not being included.
The source of irradiation can be a light source, preferably a source of ultraviolet light such a microwave electrodeless discharge tube, an arc discharge bulb, or a fluorescent discharge bulb, for example. If desired, the position of the discharge tube can be adjustable within the elliptical reflecting trough, aiding in optimization of the uniformity of the irradiation distribution on the planar target surface.